The formation of both organic and inorganic deposits in the near wellbore region of producing formations and on the tubing of a producing hydrocarbon well can be a major and costly problem. See, e.g. Allen, T. O. and Roberts, A. P., Production Operations, Vol. 2, 2nd edition, pp. 11-19 and 171-181, OGCI, Tulsa, Ok. (1982); Cowan, J. C and Weintritt, D. J., Water-formed Scale Deposits, Gulf Publishing Co., Houston (1976). The deposits can seriously impede the productivity of wells by reducing the near wellbore permeability of producing formations and progressively restrict the diameter of the tubing.
The formation of inorganic deposits, or scale, is caused by the precipitation of inorganic salts from produced water. Calcium carbonate scale is usually formed by the change in the pressure and temperature of the produced water in the near wellbore and in the production tubing. Barium, strontium and calcium sulphate scales are usually formed by the mixing of formation water and seawater injected into producing wells; the high concentration of sulphate in seawater mixes with the high concentrations of divalent cations in formation waters with the resulting precipitation of the sulphate salts. The formation of scale may be partly prevented by water shut-off treatments and the use of scale inhibitors. Once formed, scale can be removed only with some difficulty; calcium carbonate scale can be dissolved by mineral acids and barite scale can be removed by milling or scale dissolvers such as EDTA. See, e.g. Putnis, A, Putnis, C. V. and Paul, J. M., “The efficiency of a DTPA-based solvent in the dissolution of barium sulfate scale deposits”, SPE International Symp. Oilfield Chemistry, San Antonio, Tex., February 1995, SPE 29094. In extreme cases the production tubing must be removed and replaced, although the presence of radioactive scale (due to the presence of radium salts) can make scale disposal an environmental issue.
The production of hydrocarbons frequently causes the precipitation of organic precipitates such as paraffin waxes and asphaltenes. These organic precipitates are caused by changes in the pressure and temperature of the produced fluids in the near wellbore. The precipitates can be removed with solvent washes, although the disposal of the solvent after cleaning represents an increasing environmental problem.
The solubility of various inorganic and organic species can be predicted from thermodynamic models of the electrolyte solutions or hydrocarbons. See, e.g. Jasinski, R, Taylor, K. and Fletcher, P., “Calcite scaling—North Sea HTHP wells”, SPE Symp. Oilfield Scale, Aberdeen, January 1999; Calange, S., Ruffier-Meray, V. and Behar, E., “Onset crystallization temperature and deposit amount for waxy crudes: experimental determination and thermodynamic modelling”, SPE International Symp. Oilfield Chemistry, Houston, Tex., February 1997, SPE 37239. However, thermodynamic models are essentially equilibrium models and they cannot predict any details of the precipitation process such as the location of precipitation, the rate of precipitation or the degree of supersaturation that the fluid can tolerate.
Several patents and papers have described both acoustic and non-acoustic methods for sensing the formation of scale in producing hydrocarbon wells and similar environments. An acoustic method for measuring the thickness of metal oxide corrosion products on the inside of boiler tubes has been described. See Lester, S. R., “High frequency ultrasonic technique for measuring oxide scale on the inner surface of boiler tubes”, U.S. Pat. No. 4,669,310, Jun. 2, 1987. The thickness of oxide scale is determined by the time of flight of an acoustic pulse applied from the external surface of the pipe. The frequency of the acoustic pulse was 50 MHz, which enabled a scale thickness of approximately 0.1 mm to be detected. The use of an automated ultrasonic inspection system for determining the thickness of scale formation which has formed on the inside of heat transfer tubes in boilers has been described. See Okabe, Y., Iwamoto, K., Torichigai, M., Kaneko, S., Ichinari, J. and Koizumi, K., “Automated ultrasonic examination system for heat transfer tubes in a boiler”, U.S. Pat. No. 4,872,347, Oct. 10, 1989. A rotating transducer was inserted into the tubes and their diameter as a function of location determined by the reflection of sound from the scale-water interface. An acoustic wireline logging tool, Schlumberger's Cement Evaluation Tool (CET), has been used to determine the accumulation of scale on the casing of geothermal wells. See, U.S. Pat. No. 5,072,388, to O'Sullivan et al. The interfaces between the scale and wellbore fluid and the scale and the casing are determined by the transit time of the acoustic waves, which have a frequency of approximately 0.5 MHz. U.S. Pat. No. 5,092,176 disclosed a method for determining the thickness of scale on the inside of a water pipe by the attenuation of acoustic energy emitted and received by a transducer on the outside of the pipe. The optimum frequency range for the ultrasound was observed to be 3-7 MHz. For example, measurements made using ultrasound below a frequency of 3 MHz gave poor sensitivity to scale thickness and beam spreading was observed to be a problem. An acoustic method of identifying scale types and scale thickness in oil pipelines using the attenuation in the reflected acoustic signal from a tool that is moved through the inside of the pipe has been described. See, Gunarathne, G. P. P. and Keatch, R. W., “Novel techniques for monitoring and enhancing dissolution of mineral deposits in petroleum pipelines”, SPE Offshore Europe Conference, Aberdeen, September 1995, SPE 30418 (hereinafter “Gunarathne”); Gunarathne, G. P. P. and Keatch, R. W., “Novel techniques for monitoring and enhancing dissolution of mineral deposits in petroleum pipelines”, Ultrasonics, 34, 411-419 (1996) (hereinafter “Gunarathne and Keatch”). The frequency of the ultrasound used was in the range 3.5-5.0 MHz, which allowed the thickness of barium sulphate scale on steel to be measured to an accuracy of ±0.5 mm.
U.S. Pat. No. 5,661,233 (hereinafter “Spates et al.”) disclosed several acoustic-wave devices for determining the deposition of organic precipitates, such as paraffin wax, on to a sensing surface immersed in a petroleum-based fluid. The acoustic measurements were made with devices using various acoustic modes: surface acoustic waves and thickness shear, acoustic plate and flexural plate modes. The devices measured changes in the damping voltage and resonant frequency of the device as the wax precipitate formed, although no details were disclosed regarding the operating frequencies of the acoustic devices or the acoustic power they generated. Spates et al. discussed periodic cleaning of the sensing surface of the acoustic device by heating the surface to melt the paraffin wax. However, the use of acoustic energy to clean the organic precipitates from the acoustic sensor was not disclosed or suggested. Several applications of the measurement of wax accumulation were described, including location of acoustic devices on the sea floor to monitor the production of hydrocarbon from oil wells and guide well treatments. The application of a quartz microbalance to measure simultaneously mass loading and liquid properties has been described. See, U.S. Pat. No. 5,201,215; Martin, S. J., Granstaff, V. E. and Frye, G. C., “Characterisation of a quartz crystal microbalance with simultaneous mass and liquid loadings”, Anal. Chem., 63, 2272-2281 (1991) (collectively hereinafter “Granstaff and Martin”). The authors used changes in the resonant frequency and magnitude of the maximum admittance of a quartz microbalance to differentiate between changes in the mass of material deposited from a liquid and changes in the properties of the liquid (density and viscosity). The operating frequency of the quartz microbalance was close to 5 MHz, at which value the resonator was able to detect solid films of the order of 0.1 μm in thickness.
The application of an on-line quartz crystal microbalance to monitor and control the formation of organic and inorganic precipitates from hydrocarbons and water has been-described. See, U.S. Pat. No. 5,734,098 (hereinafter “Kraus et al.”). The quartz crystal microbalance consisted of a thickness-shear mode resonator and was described by Kraus et al. as being substantially similar to those disclosed by Granstaff and Martin. Kraus et al. described the use of the thickness-shear mode resonator for the on-line measurement of scaling, corrosion and biofouling in industrial processes. They also described the use of the measurement of deposit formation to determine the treatment required to correct the industrial process and prevent continual deposit formation, e.g., by use of a chemical additive such as an inorganic scale inhibitor. Although Kraus et al. described the use of a thickness-shear mode resonator to monitor deposit formation from hydrocarbons, industrial water and their mixtures (including emulsions), no disclosure or suggestion was made to the operation of these sensors in or near producing hydrocarbon wells, on either a temporary or permanent basis. Additionally, there was no disclosure or suggestion regarding the treatment of producing hydrocarbon wells, for either organic or inorganic deposit formation, on the basis of measurements made by these sensors.
A laboratory acoustic resonance technique to determine the onset of the precipitation of wax and asphaltene from produced hydrocarbons has been described. See, Sivaraman, A., Hu, Y., Thomas, F. B., Bennion, D. B. and Jamaluddin, A. K. M., “Acoustic resonance: an emerging technology to identify wax and asphaltene precipitation onset in reservoir fluids”, 48th Annual Tech. Meet. The Petroleum Society, Calgary, Canada, Jun. 8-11, 1997, paper CIM 97-96; Jamaluddin, A. K. M., Sivaraman, A., Imer, D. Thomas, F. B. and Bennion, D. B., “A proactive approach to address solids (wax and asphaltene) precipitation during hydrocarbon production”, 8th Abu Dhabi Intern. Petroleum Exhibit., Abu Dhabi, U. A. E., 11-14 Oct. 1998, SPE 49465. The spectrum of resonant frequencies of a sample of liquid hydrocarbon in a cylinder of fixed length and cross-sectional area was obtained as a function of temperature and pressure. The resonant spectra were collected using two ultrasonic transducers operating over the frequency range 0-40 kHz, although significant resonances were observed only over the frequency range 5-35 kHz. The precipitation of wax and/or asphaltene from the liquid hydrocarbon sample as the pressure and temperature of the sample was changes resulted in changes in the frequency of the resonances and changes in their amplitude. The changes in the resonant spectra were attributed to changes velocity of sound in the liquid. No reference was made to the precipitation of wax or asphaltene on the transmitting or receiving transducer.
The use of a thickness-shear mode resonator to monitor the formation of barium sulphate in samples of produced water collected at the well head has been described. See, Emmons, D. H. and Jordan, M. M., “The development of near-real time monitoring of scale deposition”, SPE Oilfield Scale Symposium, Aberdeen, 27-28 Jan. 1999 (hereinafter “Emmons and Jordan”). The resonator was immersed in a fixed volume of produced water and known amounts of soluble barium ions were added to precipitate barium sulphate scale. The resonator detected the formation of scale on its sensing surface by a decrease in resonant frequency. The amount of barium added before scale formation was detected by the resonator gave an indication of the level of inhibition in the produced water. Emmons and Jordan argued that the formation of scale by small additions of barium ions indicated the produced water was close to scaling and treatment of the well by a suitable scale inhibitor was required. Note that this method of monitoring scale formation is not an in situ method and does not measure the spontaneous formation of scale under downhole conditions of temperature, pressure, composition and flow. In addition, the resonator was not able to clean the scale from its sensing surface. A quartz crystal microbalance to monitor the formation of calcium carbonate scale under laboratory conditions has been described. See, Gabrielli, C., Keddam, M., Khalil, A., Maurin, G., Perrot, H., Rosset, R. and Zidoune, M., “Quartz crystal microbalance investigation of electrochemical calcium carbonate scaling”, J. Electrochem. Soc., 145, 2386-2395 (1998). The resonant frequency of the microbalance was 6 MHz and calcium carbonate deposition rates of 200-400 μg/cm2 per hour were measured. The calcium carbonate scale was observed to be the mineral calcite, which, with an assumed density of 2.71 g/cm3, gave deposits of 0.7-1.5 μm in thickness. The rate of scale accumulation measured by the quartz microbalance was compared with a standard electrochemical scale monitor that measured the redox current passing through an electrode as water was reduced. The decline in redox current gave an indirect measure of the decrease in the surface area of the electrode as it was covered with scale and was observed to be less sensitive to scale formation than the quartz crystal microbalance. The use of a piezoelectric quartz crystal to monitor the fouling of surfaces in a water cooling tower by inorganic scale and bacterial growth at ambient conditions has been described. See, Nohata, Y. and Taguchi, H., “An ultrasensitive fouling monitoring system for cooling tower”, Materials Performance, 34, 43-46 (1995) (hereinafter “Nohata and Taguchi”). Although Nohata and Taguchi did not specifically disclose the operating frequency of the quartz crystal, a value of about 5 MHz can be deduced from the measured accumulation rates of 1-20 μg/cm2 per day.
The use of a tuning fork for measuring the deposition of scale in a surface process system has been disclosed. See, U.S. Pat. No. 5,969,235. The accumulation of scale on the tines of the tuning fork causes a shift in the characteristic vibrating frequency of the tuning fork as measured by a suitable electronic device, such as a piezoelectric cell. The change in vibrating frequency of the tuning fork, indicating the deposition of scale, was used to control the addition of scale inhibitor to the process stream.
Non-acoustic scale sensing techniques have also been reported. A method of determining the accumulation of scale in petroleum pipelines using a heat transfer sensor has been described. See, U.S. Pat. No. 4,718,774. The scale formed on the external wall of the sensor impeded the loss of heat from a heating element in the sensor to the fluid flowing in the pipeline. The decrease in heat flow was measured by means of a temperature sensor. A wellbore scale monitor that measured the radioactivity of the radium salts precipitated with other alkaline earth metal salts has been described. See, U.S. Pat. No. 4,856,584 (hereinafter “Sneider”). Sneider discloses the use of measurements of scale radioactivity to indicate when and where the placement of scale inhibitor is required. Another scale monitoring technique is disclosed in U.S. Pat. No. 5,038,033; the radioactivity of the scale was detected by a wireline gamma ray detector, correcting for the natural gamma radiation emitted from the surrounding rock formations.
Accurately measured pressure drops over various sections of a reinjection pipeline in a geothermal power plant has been used to monitor the growth of silica scale. See, Stock, D. D., “The use of pressure drop measurements to monitor scale build-up in pipelines and wells”, Geothermal Resources Council Trans., 14, 1645-1651 (1990). The measured pressure drops across the sections of pipe produced friction factors in the range 0.1-0.2, compared to an expected value of 0.01. Cleaning the silica scale from the pipeline sections using a wire brush pig resulted in the friction factor dropping below a value of 0.06. Both laboratory and field systems to evaluate the scaling potential of oilfield brine samples by monitoring the pressure drop across a capillary tube through which the brine flows and deposits scale are being currently produced by the company Oilfield Production Analysts Ltd. (see product brochures for P-MAC 2000, 3000 and 4000 series). Three optical techniques to monitor fouling in industrial process systems were described in Flemming, H-C., Tamachkiarowa, A., Klahre, J. and Schmitt, J., “Monitoring of fouling and biofouling in technical systems”, Water Sci. Tech., 38, 291-298 (1998). The techniques consisted of measurement of the intensity of light reflected from a small optical fibre probe, the measurement of turbidity through optical windows in a flow line (using periodically cleaned windows in the flow line as a reference optical pathlength) and an infrared spectroscopy flow cell. The accumulation of deposits and the chemical nature of the deposits on the optical windows of the infrared flow cell could be determined from the infrared spectra.
A number of published reports have described the application of sonic energy for cleaning producing oil wells and equipment in similar industrial processes. A method of cleaning downhole deposits, such as tar, from producing formations and production tubing was disclosed in U.S. Pat. No. 3,970,146. However, no details were given of the power or frequency of the sound used for wellbore cleaning. A low frequency (20-100 Hz) vibrating device for cleaning deposits on the walls of casing and tubing and in formations and gravel packs was disclosed in U.S. Pat. No. 4,280,557. The vibrations were generated in the device by an orbiting mass on an unbalanced rotor, which, in turn, produced a whirling vibratory pressure of large amplitude in the fluid in the annulus. U.S. Pat. No. 4,320,528 disclosed a method of removing iron oxide corrosion products and other scaling deposits from the pipes of steam generators using a combination of high power sound and a high-temperature solvent (e.g., sodium EDTA, citric acid and a corrosion inhibitor). The acoustic transducers operated in the frequency range 2-200 kHz and generated an output acoustic power greater than 0.2 W/cm2, a value which is above the cavitation threshold of aerated water at ambient pressure and temperature. See, Esche, R., “Untersuchung der Schwingungkavitation in Flüssigkeiten”, Acustica, 2, AB208-218 (1952); Mason, T. J. and Lorimer, J. P., Sonochemistry: Theory, Applications and Uses of Ultrasound in Chemistry, p 31, Ellis Horwood, Chichester, UK (1988). The transducers were located permanently on the outside of the heat exchanger tubes. U.S. Pat. No. 4,444,146 disclosed an ultrasonic method to clean the fouled surfaces of submerged structures, such as the hulls of ships. The ultrasonic cleaner consisted of two ultrasonic transducers focussed on a small area of surface to be cleaned. The transducers operated at slightly different frequencies, typically in the range 180-210 kHz; no details were disclosed on the acoustic power required to clean the fouled surfaces. UK Patent Application 2 165 330 A (hereinafter “D'Arcy et al.”) disclosed a method of cleaning underwater structures to depths of up to 1000 meters using focussed ultrasound in the frequency range 40-100 kHz. The ultrasound was generated and focussed using an array of transducers located on the concave surface of a spherical cap. The density of acoustic power at the focal point of the array of transducers was stated to be about 500 W/cm2, a value that is approximately 3 orders of magnitude above the cavitation threshold of water at ambient pressure. D'Arcy et al. suggested the high power acoustic array could be used to clean the base of oil production platforms. U.S. Pat. No. 5,184,678 disclosed the design of a high power acoustic logging tool to stimulate fluid production from oil wells. The acoustic power was provided by pulsed magnetostrictive transducers operating in the frequency range 5-30 kHz and emitting an acoustic power density of up to 1 W/cm2. The tool was designed to give a stand-off from the treated formation of 0.2-0.5λ, where λ is the wavelength of the sound in the borehole fluid. The treated formations were exposed to the acoustic power for periods of 5-60 minutes. According to U.S. Pat. No. 5,184,678, the applied ultrasound reduced the viscosity of the fluid in permeable formations and fluidised the particulate matter, thus facilitating its removal.
It has been shown that ultrasound applied at a frequency of 10 kHz could remove asphaltene deposits from a sand pack saturated with both water and kerosene at ambient pressure. See, Gollapdi, U. K., Bang, S. S. and Islam, M. R., “Ultrasonic treatment for removal of asphaltene deposits during petroleum production”, SPE International Conf. Formation Damage Control, Lafayette, La., February 1994, SPE 27377. Although the acoustic power applied to the sand packs during cleaning was not measured, the ultrasonic transducer could generate a maximum output acoustical power of 250 W. The authors discussed the role of acoustic cavitation in the cleaning process and acoustic cavitation was undoubtedly achieved at the power settings reported. The asphaltene deposits were observed to be removed significantly more efficiently by the ultrasound in kerosene than in water. It has been demonstrated under laboratory conditions that the damage caused to permeable formation by the invasion of clay particles from drilling fluids can be partially removed by the application of high power ultrasound. See, Venkitaraman, A., Roberts, P. M. and Sharma. M. M., “Ultrasonic removal of near-wellbore damage caused by fines and mud solids”, SPE Drilling & Completions, 10, 193-197 (1995). Two ultrasonic transducers were used; one was a high power ultrasonic horn operating at a frequency of 20 kHz with an output power of up to 250 W and the other was a low power transducer operating over frequency range 10-100 kHz. The same authors subsequently evaluated the application of high power ultrasound under laboratory conditions for the removal of organic deposits and formation damage caused by the invasion of drilling fluid filtrate containing water-soluble polymers. See, Roberts, P. M., Venkitaraman, A. and Sharma, N. M., “Ultrasonic removal of organic deposits and polymer induced formation damage”, SPE Formation Damage Control Symp., Lafayette, La., February 1996, SPE 31129. Using the same ultrasonic transducers, it was demonstrated that polymer-induced formation damage was considerably more difficult to remove than the damage caused by clay fines. However, formation damage resulting from the precipitation of wax in the test core samples could be removed by sonication when the core samples were soaked in a suitable solvent.
U.S. Pat. No. 5,595,243 disclosed the use of a general purpose acoustic cleaning tool for improving the near wellbore permeability of producing formations by redissolving or resuspending restricting materials. The cleaning tool was reported to generate acoustic power densities of up to 2 W/cm2, which is above the cavitation threshold for water at ambient temperature and pressure. See, U.S. Pat. No. 4,280,557. The tool, which consisted of an array of air-backed high power acoustic transducers of the type described by Widener, was designed to be deployed into the well on a wireline cable. See, Widener, M. W., “The development of high-efficiency narrow-band transducers and arrays”, J. Acoust. Soc. Amer., 67, 1051-7 (1980); Widener, M. W., “The development of a deep submergence air-backed transducer”, J. Acoust. Soc. Amer., 80, 1852-3 (1986). The transducers described by Widener would be expected to operate in the frequency range 10-100 kHz. U.S. Pat. No. 5,676,213 disclosed the use of high power ultrasound to remove the filter cake formed by the drilling fluid during the drilling of a well in order to measure the pressure in permeable formations. The high power ultrasound was generated by a focussing transducer operating in the frequency range 100-500 kHz and capable of operating at a peak input power of up to 1 kW. U.S. Pat. No. 5,727,628 disclosed an ultrasonic tool for cleaning producing wells. The wireline-deployable tool consisted of an array of magnetostrictive transducers operating in the frequency range 18-25 kHz (preferably at 20 kHz) and emitting an acoustic power density in the range 8-12 W/cm2. The tool was also equipped with a pump to remove the debris of the fouling deposits disaggregated by the ultrasonic tool. U.S. Pat. No. 5,735,226 disclosed a method to prevent the fouling of ships and other marine structures by the use of ultrasound over the frequency range 20-60 kHz. One demonstration of the technique was the location of a number of ultrasonic transducers on the hull of a ship over a period of 4 months. Over this time period the transducers, which were powered intermittently, gave effective relief from marine fouling. U.S. Pat. No. 5,735,226 revealed no details of the power consumption of the transducers, but one embodiment of the invention consisted of the array being powered by a 9 volt battery. U.S. Pat. No. 5,889,209 disclosed the use of high power ultrasound to prevent biofouling of chemical sensors used in aquatic environments. The ultrasound was generated by a transducer operating in the frequency range 10-100 kHz and yielding a sufficient power density (>0.1-1 W/cm2) to drive acoustic cavitation. U.S. Pat. No. 5,889,209 disclosed the use of the acoustic cleaning technique to maintain the performance of a dissolved oxygen sensor located in microbiologically active water for seven days. The transducer was located over the range 4-10 mm from the active membrane of the oxygen sensor and activated for a time period of 6-90 seconds over a time interval of 5-120 minutes.
Several papers and patents have reported on the use of high power ultrasound to accelerate the dissolution of scale by chemical scale dissolvers applied in pipelines and producing oil wells. Paul, J. M. and Morris, R. L., “Method for removing alkaline scale”, International Patent Application WO 93/24199, 9 Dec. 1993 describes the use of low frequency (1.5-6.5 kHz) sonic energy to accelerate the dissolution of alkaline earth metal scales using scale dissolving solutions (typically containing the chelating agents EDTA or DTPA). Gunarathne, and Gunarathne and Keatch have shown that the application of low-power ultrasound can increase the rate of dissolution of barium sulphate scale using commercially available scale dissolvers; the power density was claimed to be below that required to cavitate the scale dissolving solution.
In conclusion, there appears to be no prior art that teaches or suggests either an acoustic scale sensor or an acoustic cleaning device located permanently or quasi-permanently in a well producing hydrocarbons. There appears to be no prior art of teaches or suggests the concept of a sensor for hydrocarbon wells to monitor the formation of inorganic or organic scales, biofouling or corrosion and initiate a cleaning action. Additionally, there appears to be no prior art that teaches or suggests an on-line deposits monitoring and cleaning device located on the surface facilities of a producing oil well using an ultrasonic transducer operating in its longitudinal mode and coupled to the produced fluids using a coupling material, such as an acoustic horn, to which the deposits adhere.